Method of fabrication of an infrared radiation detector and infrared detector device

ABSTRACT

A method of fabricating an infrared detector, a method of controlling the stress in a polycrystalline SiGE layer and an infrared detector device is disclosed. The method of fabricating includes the steps of forming a sacrificial layer on a substrate; patterning said sacrificial layer; establishing a layer consisting essentially of polycrystalline SiGe on said sacrificial layer; depositing an infrared absorber on said polycrystalline SiGe layer; and thereafter removing the sacrificial layer. The method of controlling the stress in a polycrystalline SiGe layer deposited on a substrate is based on varying the deposition pressure. The infrared detector device comprises an active area and an infrared absorber, wherein the active area comprises a polycrystalline SiGe layer, and is suspended above a substrate.

This is a continuation of application Ser. No. 09/702,501 filed on Oct.31, 2000 now U.S. Pat. No. 6,274,462, which is a continuation ofapplication Ser. No. 09/049,797 filed on Mar. 27, 1998 now U.S. Pat. No.6,194,722.

REFERENCE TO RELATED APPLICATIONS

This application claims priority to European patent application number97870044.1 filed on Mar. 28, 1997.

OBJECT OF THE INVENTION

The present invention is related to a method of fabrication for aninfrared radiation detector, and more particularly an infrared sensitivebolometer, using micro-machining techniques.

The present invention is also related to the infrared detector itself.

Finally, the present invention is related to a specific use of surfacemicro-machining techniques.

BACKGROUND OF THE INVENTION

A large number of infrared detection methods exist, each being based ona different working principal. Infrared detectors are being used in alarge number of applications.

The present invention focuses on the group of detectors where the energyof the absorbed infrared radiation raises the temperature of thedetecting element thereby changing its electrical conductivity. Thesedetectors, known as bolometers, are fabricated from different materialslike metals, permalloy, vanadium oxide, or (poly-crystalline) silicon.

In order to obtain a high performance, two points are important:

-   -   1) the total thermal conductance G from the resistor to the        substrate must be low, so as to maximise the temperature        increase for a given amount of energy deposited on the detector;        and    -   2) the absolute value of the temperature coefficient of        resistance (TCR) α (i.e., the percent variation of the device        resistance for a temperature increase of 1 K) must be large.

The first point is related to the geometrical structure of the detectorand to the thermal properties of the material(s) forming it, and thesecond one is related only to the electrical properties of the activematerial.

With technologies suggested in the state of the art of micro-machininggood thermal insulation is obtained in two different ways either bymicro-machining an electrically and thermally insulating membrane anddepositing the active material onto it, either by micro-machiningstructures suspended over the substrate directly using the activematerial. This last approach is more simple and straightforward butrequires an active material with low thermal conductance and withmechanical properties adequate for micro-machining. Until now, this isapplied only to poly-crystalline silicon (poly-Si) bolometers.

An example of the first approach is reported in document WO-A-9313561which describes a method for fabricating an integrated infraredsensitive bolometer having a polycrystalline element whereby an oxideregion deposited on silicon nitride covered with a first polysiliconlayer which is etched to provide a location for a bolometer element. Asecond polysilicon layer is deposited and doped to achieve a desiredtemperature coefficient of resistivity of 1 to 2%/° C. The secondpolysilicon layer forms an infrared sensitive element over the oxideregion. Openings are etched in the infrared sensitive element to permitan etchant to remove the oxide region resulting in the sensitive elementbeing suspended over the cavity. The thermal conductance is determinedby the thermal conductivity of poly-Si and by the shape of the etch ofthe first poly-Si layer.

An example of the second approach is described in the document “InfraredFocal Plane Array Incorporating Silicon IC Process Compatible Bolometer”of Tanaka, et al. published in IEEE Transactions on Electron Devices,Vol. 43, No. 11, November 1996 which describes a 128×128 elementbolometer infrared image sensor using thin film titanium. The device isa monolithically integrated structure with a titanium bolometer detectorlocated over a CMOS circuit that reads out the bolometer's signals. Byemploying a metallic material like titanium and refining the CMOSreadout circuit, it is possible to minimize 1/f noise. Since thefabrication process is silicon-process compatible, costs can be keptlow.

The article “The Growth and Properties of Semiconductor Bolometers forInfrared Detection” of M. H. Uniwisse, et al in SPIE Vol. 2554/43describes how to develop bolometer arrays from semiconductor materials,such as the amorphous and microcrystalline phases of Si, Ge, and SiGe.In this work, the use of amorphous and microcrystalline SiGe:H issuggested in order to reduce the large 1/f noise and the largeresistivity of amorphous silicon. No use of the thermal properties ofSiGe is mentioned.

The article “Thin Film Boron Doped Polycrystalline Silicon Germanium forthe Thermopiles” of P. Van Gerwen, et al. in the 8^(th) InternationalConference on Solid-State Sensors and Actuators, and Eurosensors IX,Stockholm, Sweden, Jun. 25-29, 1995 describes the use of polycrystallinesilicon-germanium for thermopiles instead of polysilicon. Thermopilescan be used for infrared detection if incident infrared is absorbed byan absorption layer near one junction which will heat up. The otherjunction is put on a heat sink, which will not heat. The temperaturedifference between the two junctions creates a voltage related to theabsorbed infrared. These thermopiles are fabricated according to bulkmicro-machining techniques.

Double-sided processing and special handling requirements though makebulk micro-machining incompatible with standard IC fabricationtechniques.

U.S. Pat. No. 5,367,167 is describing a bolometer for detectingradiation in a spectral range including an integrated circuit substrateand a pixel body spaced from the substrate. The pixel body comprises anabsorber material such as titanium for absorbing radiation in thespectral range. In addition, a variable resistor material which is theactive element made of amorphous silicon is formed over an insulatinglayer.

The article “Thermal stability of Si/Si_(1-x)Ge_(x)/Si heterostructuresdeposited by very low pressure chemical vapor deposition” published inApplied Physics Letter, vol. 61, No. 3, of 20 Jul. 1992, pp; 315-316describes structures using crystalline SiGe deposited on crystalline Si.More particularly, this document is related to the study of the thermalstability of metastable Si/Si_(1-x)Ge_(x)/Si strained structuresdeposited by very low pressure chemical vapor deposition.

OBJECTS OF THE INVENTION

The first object of the present invention is to provide a method offabricating an infrared radiation detector, and more particularly aninfrared sensitive bolometer, having improved thermal insulation.

A second object of the present invention is to suggest a method offabricating a bolometer which can be arranged in arrays of pixels forapplications such as (night) cameras, for instance.

A third object of the present invention is to disclose the fabricationof an infrared radiation detector with a material having lower thermalconductance compared to state of the art detectors but still compatiblewith standard IC technologies and having electrical and mechanicalproperties which are at least similar to those of the state of the art.

SUMMARY OF THE PRESENT INVENTION

As a first aspect, the present invention is related to a method offabricating an infrared sensitive bolometer comprising the steps:

-   -   forming a sacrificial layer on a substrate;    -   patterning said sacrificial layer;    -   establishing, through depositing or growing, an active layer        made of polycrystalline SiGe on said sacrificial layer;    -   depositing an infrared absorber on said polycrystalline SiGe        layer;    -   removing the sacrificial layer.

Preferably, the method further comprises, after establishing an activelayer, through depositing or growing, the steps of:

-   -   patterning said polycrystalline SiGe layer, thereby forming an        active area and supports of the detector;    -   performing a high doping of the supports and a moderate doping        of the active area;    -   depositing an infrared absorber on said polycrystalline SiGe        layer.

In this way, a poly-SiGe resistor thermally insulated from the substratewill be formed. Infrared radiation will generate an increase oftemperature which will in turn originate a decrease in resistance.

The sacrificial layer can be defined as a layer used only in order torealize the purpose of thermal insulation of the bolometer. In fact, thebolometer should be thermally insulated, otherwise, its temperature willbe no longer controlled by the incident radiation. Thus, the structureis built on the top of the sacrificial layer and is connected to thesubstrate through the supports. After removing the sacrificial layer,the supports will be the thermal path between the device and thesubstrate. Hence, the thermal conductance of the supports play animportant role in insulating the device and therefore should beminimized.

The active area can be understood as the area of the bolometer, whichworks as a detector portion.

Patterning should be understood as forming structures on/in differentlayers on/in the substrate according to the techniques known in themicroelectronics and micromachining such as photolithography.

Preferably, the step of patterning said sacrificial layer comprises thesteps of defining anchor points.

Furthermore, the method of fabrication according to the presentinvention can also include the step of depositing one or moreintermediate layer(s) in said substrate prior to depositing saidsacrificial layer and wherein the intermediate layer(s) comprise atleast an etch stop layer.

Preferably, the substrate is a Si substrate and the active layer is madeof polycrystalline Si_(70%)Ge_(30%).

According to one preferred embodiment, the regions having thesacrificial layer can be formed with a porous silicon region.

According to another preferred embodiment, these regions having thesacrificial layer can be created by depositing and patterning aninsulating layer made of an oxide.

The steps of high doping of the supports and moderate doping of theactive area can be achieved by means ion implantation.

The polycrystalline SiGe layer is preferably deposited by chemicalvapour deposition at atmospheric pressure or at a reduced pressure.

The infrared absorber can be created according to a first preferredembodiment with a stack of layers comprising a metallic layer, aninsulating layer and a semi-transparent metallic layer. According to asecond embodiment, the infrared absorber only comprises a metallic layerand an insulator of appropriate thickness.

According to a third preferred embodiment, the infrared absorbercomprises at least a semitransparent metallic layer

As a second aspect, the present invention is related to a method ofcontrolling stress in a polycrystalline SiGe layer deposited on asubstrate such as a silicon oxide substrate by varying the depositionpressure and/or the annealing temperature.

As a third aspect, the present invention is related to an infraredsensitive bolometer comprising an active area and an infrared absorber,wherein said active area comprises a polycrystalline SiGe layer, and issuspended above a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b represent the cross-section of two possibleembodiments of structures for a bolometer achieved by the method of thepresent invention while FIG. 1 c represents the top view of bothembodiments shown in FIGS. 1 a and 1 b.

FIG. 2 shows a schematic representation of a pixel and its externalelectronics of a camera according to one embodiment of the invention.

FIG. 3 represents the behaviour of stress of a polycrystalline SiGe filmdeposited on a carrier as a function of the annealing temperature fortwo different deposition pressures.

FIG. 4 represents the dependence of the power dissipated in a structuresimilar to the structure described in FIG. 1C on the temperature rise ofthe bolometer. The slope of the curve gives the thermal conductance(W/ΔT) of the bolometer, wherein the dots refer to a structure usingpoly-Si as active material while the small squares refers to a structureusing poly-SiGe as active material according to the invention.

FIGS. 5 a to 5 f represent the side view and the top view for severalsteps of the fabrication process of a bolometer according to a preferredembodiment of the present invention.

FIGS. 6 a & 6 b show SEM pictures of two micromachined bolometersaccording to several embodiments of the invention having lateraldimensions of 50 μm(a) and 25 μm(b) respectively and wherein thethickness of the poly SiGe layer is 1 μm, the width of the supports is 5μm and 0.6 μm for the 50×50 μm(a) bolometer(s) and the 25×25 μm(b)bolometer respectively.

FIG. 7 displays the experimentally measured. absorbance of an infraredabsorber used in one embodiment of the invention, versus the wavelengthof the radiation.

FIG. 8 represents the experimentally measured responsivity as a functionof the supply voltage of a bolometer fabricated according to theinvention and wherein the responsivity is expressed in Volt/Watt andrepresents the voltage generated per unit of power incident in thebolometer.

FIG. 9 displays the measured frequency dependence of the total noise ofa bolometer according to one embodiment of the invention as shown inFIG. 6 b over the range extending from 1 Hz to 100 KHz.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

In an aspect of the present invention, a method of fabricating aninfrared bolometer is described, which has an improved thermalinsulation behaviour. Furthermore, a bolometer fabricated according tothe method of the invention is described.

In order to evaluate a material for the realisation of a bolometer, itis necessary to understand how its properties influence the responsivityand the noise of the bolometer itself.

According to the invention, use is made of an alloy comprisingpolycrystalline Silicon Germanium (poly-SiGe).

This material has lower thermal conductance than polycrystallinesilicon, and is compatible with the standard IC technology. It also haselectrical and mechanical properties which are superior to those ofpolycrystalline silicon.

Several possible structures of bolometers are shown in the FIGS. 1 a, 1b, and 1 c.

FIG. 1 a represents a bolometer wherein porous silicon regions 11 havebeen formed on a substrate 10, while FIG. 1 b represents the depositionand the patterning of an insulating layer 12 on a substrate 10. In bothcases, a polycrystalline SiGe layer 13 is then established, bydepositing and patterning, in order to form the active area 16 and thesupports 15 connected to the substrate at the anchor points 14 asrepresented in FIG. 1 c.

The particular shape of FIG. 1 c has to be considered only as an exampleand that the present invention is not limited to this particular shape,but covers all similar structures, characterised by long and thinsupports.

For the purpose of teaching of the invention, an important applicationof bolometers of the invention is described below. One of the mostimportant applications of the bolometers is the realisation of aninfrared camera. Said camera is an array of pixels, each containing abolometer and some electronics.

A schematic view of a pixel is shown in FIG. 2. The bolometer isrepresented by the reference 21, and is connected to a CMOS switch 22,built in the substrate, while the signal and gate lines 23 and 24 go tothe external read-out. Externally to the array, a voltage waveformgenerator 25 and a load resistor 26 are provided. During calibration,the CMOS switch 22 is closed, a short voltage pulse is sent to thebolometer, and the voltage drop across the load resistance is measured.This operation is repeated in presence of infrared radiation. A signalis defined as the difference of the voltage across the load resistormeasured with and without infrared radiation. Note that in a normaloperation, the duration of the voltage pulse is shorter than the thermaltime constant of the bolometer, and does not create a temperatureincrease.

The volt responsivity , i.e. the signal per unit incident power of abolometer is given by:$\Re = \frac{{\alpha }\eta\quad{VR}_{L}R_{B}}{\left( {R_{L} + R_{B}} \right)^{2}G}$where R_(L) and R_(B) are the load resistance and bolometer resistance,respectively.

G is the thermal conductance of the supports towards the substrate. Itdepends mainly on the thermal conductivity of the material and thegeometry of the supports.

An appropriate expression of the thermal conductance (for long supports)is $G = {g\frac{wt}{l}}$where the w, t and l are the width, the thickness, and the length of thesupports respectively. It is clear from the above expression that thethermal conductance can be minimized by using thin and long supports.

It is also evident that in order to obtain a large responsivity, theapplied voltage V, the absolute value of the temperature coefficient ofresistance α and the infrared emissivity η must be large, while thetotal thermal conductance toward the support, G, must be minimised.

The noise which affects this responsivity signal has two contributions:external (from read-out electronics, load resistor, amplifiers . . . )and internal (due to the bolometer itself). External noise can bereduced by design optimisation and is, to a large extent, independent ofthe bolometer characteristics. Concerning the bolometer noise, therelevant quantity is not the noise itself but the noise equivalent power(NEP), i.e. the power which generates a signal equal to the noise.

Three contributions should be considered to the noise equivalent power:the Johnson noise, the temperature fluctuation noise and the lowfrequency noise typical of the active material.

For the case of Johnson noise, it can be defined as:${NEP}_{J} = {\frac{\sqrt{4{KTR}_{B}}}{\Re}\Delta\quad{fe}}$

It can be minimised by reducing the bandwidth Δf_(e) with appropriateactions on the electronics or by using a low value for the bolometerresistance.

For the case of thermal fluctuation noise, it can be defined as:NEP_(th)=√{square root over (4kGT²)}Δf_(t) wherein Δf_(t) is a thermalbandwidth of the order of the inverse of the thermal time constant ofthe bolometer.

The l/f noise is given by${V_{n,1}/f} = {k\quad V_{b}\sqrt{\frac{\rho}{WLTf}}}$wherein the coefficient k depends of the microcrystalline structure ofthe material, V_(b) is the voltage across the terminals of thebolometer, ρ is the resistivity, W, L and T are the width, length andthickness of the pixel respectively. It should be noted that also thiscontribution to the noise is, as the Johnson one, minimized by using lowvalues of the bolometer resistance. Furthermore, the l/f noise increasesproportionally with the baising voltage which is not the case for theJohnson noise.

It is evident that the conditions which maximise the responsivity alsominimise the NEP. In particular, as far as the material properties areconcerned, one needs low G and large |α|. These properties for thepoly-SiGe used in the present invention will be discussed further.

Meanwhile, the influence of l/f noise will be discussed together withthe responsivity results.

Thermal Conductance and Stress Properties of the Material Used in theBolometer of the Invention

The possibility of realising a good thermal insulation depends on thegeometry of the structure and on the thermal conductance of thematerial. It is known that in order to realise micro-machined structuressimilar to that of FIG. 1 it is important to use films with low, andpossibly tensile, internal stress.

In FIG. 3, the behaviour of the stress in a poly-SiGe film having athickness of 1.0 μm deposited onto an oxide is shown as a function ofthe annealing temperature.

The material grown at atmospheric pressure 32 has a relatively lowcompressive stress, and this stress is reduced to zero after anannealing at 900° C. for 30 minutes. For comparison, poly-Si grown bylow pressure chemical vapour deposition at 620° C. exhibits an internalcompressive stress larger than 300 MPa, which is annealed out at atemperature of 1050° C.

The material grown at reduced pressure 31 (lower than the atmosphericpressure e.g. 40 torrs) has a tensile internal stress, which is almostinsensitive to annealing. The above observations indicate that theinternal stress of poly-SiGe can be tuned by appropriate choice ofgrowth/deposition conditions and annealing temperatures. This allows oneto choose the stress of the active layer so as to compensate the one ofthe layers forming the infrared absorber. By this procedure, low stressand mechanically stable structures can thus be obtained. Except that inthe case of an infrared absorber with very low tensile stress, thiscompensation procedure requires low annealing temperature or even noannealing at all.

Furthermore, the thermal conductivity of poly-SiGe depends on theconcentration of germanium atoms and has a minimum at a germaniumconcentration of 30%. In FIG. 4, the temperature increase ΔT obtained bydissipating the power W in a structure similar to that of FIG. 1C isreported. The dots refer to a structure made in poly-Si, the squares toa structure made of poly SiGe. The two structures are identical in shapebut the thermal insulation is almost 6 times larger for poly SiGe. Usinga finite element model for thermal conductivity, the values 17 W/mK and2.5 W/mK are deduced for the thermal conductivity of the two materials.This reduction of thermal conductivity has obvious beneficial effects onthe responsivity and on the noise equivalent power of the bolometer.

Electrical Properties of the Materials used in the Bolometer of theInvention

In order to use a material for realising a bolometer, it is necessary tocontrol by appropriate doping, both its conductivity and the dependenceof conductivity on temperature. In Table I, the conductivity and thetemperature coefficient of resistance are reported for differentimplanted doses of boron. The annealing temperature after ionimplantation is also indicated.

It is possible to note that at low and medium doses, the temperaturecoefficient of resistance is large, as necessary for a well performingbolometer. The resistivity of highly doped material is very low. Thisallows one to regard the highly doped supports as conducting leads. Theattention is pointed to the fact that activation of dopants is obtainedalso at temperatures as low as 650° C. This implies that a low thermalbudget process is possible.

In conclusion, it can be stated that these properties show thatpoly-SiGe is a suitable material for surface micro-machiningapplications.

Detailed Description of an Example According to a Preferred Embodimentof the Present Invention

In the following, the description is focused to the case of asacrificial layer realised with an oxide, as represented in FIG. 1 b. Anexample of thermal oxide can be TEOS. The process described hereafterapplies with minor modification to a sacrificial layer made of poroussilicon as represented in FIG. 1 a.

The substrate is a silicon wafer, said substrate being already processedin order to form the appropriate CMOS switches of the infrared cameraand covered with standard passivation layers.

In the following, the realisation of the bolometer is explained and nodetails will be given about the connection of the bolometer to theunderlying electronics and to the external circuitry as they are notrelevant to the present invention and can be made according to theknowledge of the man skilled in the art.

It is important to note at this stage that the preparation of bolometersusing an already processed substrate is made possible by the relativelylow temperature at which the bolometer is prepared.

As shown in FIG. 5 a, a thin layer 42 (e.g. 100 nm thick) is depositedonto the substrate 41, in order to serve as etch stop during the etchingof the sacrificial layer 43. This layer 42 can be according to a firstpreferred embodiment a silicon-rich silicon nitride or any othermaterial which is resistant to the etching agent of the sacrificiallayer. According to another preferred embodiment, the thin layer 42 canbe an undoped poly-Si layer.

The sacrificial layer 43 (1 or 2 μm of TEOS) is then deposited andpatterned by standard lithography. Typical lateral dimension of theremaining TEOS regions after patterning vary from 20 μm to 50 μm.

Subsequently, as represented in FIG. 5 b, a poly-SiGe active layer 44(0.5 μm or 0.3 μm thick) is established, by depositing in a chemicalvapour deposition system at a temperature comprised between 600° C.-700°C. and preferably around 650° C. at atmospheric or at reduced pressure.

Gases used for the deposition are diclorosilane and germane. Theirrelative proportion is chosen in such a way that the concentration ofgermanium in the film is about 30%. This is the concentration whichminimises the thermal conductivity.

Using the above process gas, the nucleation of poly-SiGe on oxides isslow, then a thin (30 Å) nucleation layer of poly-Si is used. It is alsopossible to use a mixture of silane and germane. In this case, nonucleation layer will be necessary.

After the poly-SiGe deposition, a thin layer of silicon nitride 45 isdeposited. It will serve for electrical insulation of the absorber fromthe active element.

The stack of these two layers is then patterned. The holes 46 on theactive layer will be useful during the sacrificial layer etching. Thecontacts 47 drawn in FIG. 4 b are large and useful for testing singledevices. When the device is inserted in a matrix their dimension can bereduced.

In FIG. 5 c is shown that the contacts 47 and the supports 48 areimplanted with a high boron dose and the active area with a moderatedose. Instead of implanting the active area by a low doping dose, it isalso possible to dope it at a desired level during the deposition ofpoly-SiGe by adding appropriate gases to diclorosilane and germane. Anannealing for dopant activation and diffusion is performed. By means ofthe above dopings, the supports act as electrical contacts and theconductivity and the TCR of the active area is tuned to the desiredvalue.

The construction of the bolometer proceeds by the deposition andpatterning of the absorber 49 as represented in FIG. 5 d. At this stage,either vias to the underlying electronics are opened or metal contact410 are deposited for test purposes (see FIG. 5 e).

The construction of the bolometer is completed by the removal of thesacrificial layer using standard micro machining techniques (see FIG. 5f). The absorber can be composed, for example, of a triple layer:starting from the silicon nitride, a metal reflector, an insulatinglayer about 1.5 μm thick, and a semitransparent metal are formed. Otherpossibilities for preparing the absorber exists and can be used.

Another absorber can be made as follows: an infrared absorber having aTi layer of 0.2 μm thick, a polyimide (Hitachi polyimide) layer of 1.4μm thick and a semitransparent NiCr layer having a thickness of 5 nm issuggested.

The Ti layer can be up to 1 μm thick in order to have better reflectorproperties. However, if the thickness of this Ti layer is too large, thestress will also be too high and the device will crack.

The thickness of the polyimide define the wavelength maximal absorption.This means that if the thickness changes, the wavelength will alsochange.

The formula $T = \frac{\lambda}{4n}$is determining, wherein according to this example, λ=10 μm and n=1.8which will give a thickness of 1.4 μm.

The NiCr layer can have a thickness of 2 to 5 or 10 nm or even more. Forinstance, for λ=10 μm, a thickness of 5 nm will give 100% absorbancewhich corresponds to the conditions for destructive interference.

In FIG. 6, SEM pictures of two micromachined bolometers are represented.These bolometers have lateral dimensions of 50 μm and 25 μm respectivelywith a thickness of the poly SiGe layer of 1 μm. For the small devices,the support width is only 0.6 μm, and the thermal conductance in vacuumis only 10⁻⁷ W/K.

In FIG. 7, the absorbance of the infrared absorber as a function of thewavelength is represented, it is shown that an absorbance of 0.7 can beobtained in the 8-14 μm region.

In FIG. 8, the responsivity measured as a function of the applied dcbias voltage is reported. Considering that in camera operation, pulsedbias is used, and the voltage can be increased up to 5 V, responsivitiesin excess of 10⁵ V/W can be obtained.

In FIG. 9, the bolometer noise as a function of frequency is reported. Afrequency independent detectivity cannot be defined. Having in mindinfrared camera application, considering a frequency bandwidth extendingfrom 10 to 100000 Hz, the average noise and detectivity in this rangecan be computed.

It is intended that the foregoing detailed description be regarded asillustrative rather than limiting and that it is understood that thefollowing claims, including all equivalents, are intended to define thescope of the invention.

TABLE I Values of resistivity and temperature coefficient of resistancefor different implanted doses of boron and for different annealingtemperature. Resistivity α T_(ann) = 650 T_(ann) = 850 T_(ann) = 850Dose ° C. ° C. ° C. 5 10¹² — 400 Ωcm −4.3% (B/cm²) 3 10¹³ 21 Ωcm 30 Ωcm−2.0% (B/cm²) 10¹⁶ 7 10⁻³ Ωcm 3 10⁻³ Ωcm — (B/cm²)

1. A method of fabricating an electrical device comprising: depositing asacrificial layer comprising a first material on a substrate; depositinga second material on the substrate; contemporaneously forming an activelayer portion and at least one connector portion from the secondmaterial, wherein the connector electrically couples the active layerportion with circuitry included on the substrate; and removing at leasta portion of the sacrificial layer.
 2. The method of claim 1, whereindepositing a sacrificial layer includes forming a sacrificial layer onthe substrate and patterning the sacrificial layer.
 3. The method ofclaim 1, wherein the second material consists essentially of a singlematerial.
 4. The method of claim 3, wherein the single materialcomprises polycrystalline silicon.
 5. The method of claim 3, wherein thesingle material comprises silicon-germanium.
 6. The method of claim 3,wherein forming the active layer portion and the at least one connectorportion comprises: patterning the second material, so as tocontemporaneously form the active area portion, the at least oneconnector portion and at least one support of the active layer.
 7. Themethod of claim 6, wherein forming the active layer portion and the atleast one connector portion further comprises: doping the active layerportion and the at least one connector portion after pattering thesecond material.
 8. A method of fabricating an electrical devicecomprising: depositing a sacrificial layer on a substrate; forming anactive area; forming at least one connector, the connector electricallycontacting the active area with circuitry included on the substrate; andremoving at least a portion of the sacrificial layer, wherein the activearea and the connector consist essentially of a single material, and theactive area is suspended above the substrate using one or more supportsConned with the active area.
 9. The method of claim 8, wherein formingthe active area and forming the at least one connector is performedsubstantially simultaneously.
 10. The method of claim 9, wherein formingthe active area and forming the at least one connector comprises:patterning the active area and the at least one connector.
 11. Themethod of claim 10, wherein forming the active area and forming the atleast one connector further comprises: doping the active area and the atleast one connector.
 12. The method of claim 9, further comprising:doping the electrical device after forming.
 13. The method of claim 12,wherein doping includes performing doping of the at least one connectorwith a different doping concentration than the active area.
 14. Themethod of claim 13, wherein doping the at least one connector includeshighly doping the at least one connector.
 15. The method of claim 12,wherein doping includes performing doping of the active area and the atleast one connector.
 16. The method of claim 11, wherein doping of theactive area and the at least one connector includes moderately dopingthe active area and the at least one connector.
 17. The method of claim8, wherein the electrical device comprises an infrared device.
 18. Themethod of claim 8, wherein the electrical device comprises a detector.19. The method of claim 8, wherein the single material comprisespolycrystalline.
 20. The method of claim 8, wherein the single materialcomprises silicon-germanium.
 21. The method of claim 8, wherein removingincludes removing substantially all of the sacrificial layer.
 22. Themethod of claim 8, wherein depositing the sacrificial layer comprises:forming a sacrificial layer on the substrate; and patterning thesacrificial layer.
 23. The method of claim 8, wherein the active areacomprises a material made of polycrystalline Si_(70%)Ge_(30%).
 24. Themethod of claim 23, wherein the active area is deposited by chemicalvapor deposition.
 25. The method of claim 23, wherein the active area isdeposited at or below atmospheric pressure.